A conventional screw compressor is composed of, as shown in a developed view of FIG. 7, a screw rotor 40, a pair of gate rotors 44 (only one rotor is shown) engaging with the screw rotor 40 interposed therebetween, and a cylinder 41 for housing the screw rotor 40 in a rotatable state (see Patent Document JP 11-248264 A). The screw rotor 40 rotates in an arrow A direction.
A compression chamber 43 is formed between the screw rotor 40 and the cylinder 41. More specifically, the compression chamber 43 is tightly closed by engagement between a screw groove 40a of the screw rotor 40 and a tooth section 44a of the gate rotor 44.
The cylinder 41 has an economizer port 42 for jetting a refrigerant into the compression chamber 43.
The economizer port 42 does not yet communicate with the compression chamber 43 immediately after closing the compression chamber 43. The economizer port 42 opens the compression chamber 43 after starting to compress the refrigerant, as shown by a dotted line in FIG. 2.
In the conventional screw compressor, the economizer port 42 communicates with the compression chamber 43 when the inner pressure of the compression chamber 43 is high after starting to compress the refrigerant. Therefore, a pressure in the economizer port 42, which communicates with the compression chamber 43, is also high (e.g., the average pressure in the economizer port 42 becomes about 7 kg/cm2, as shown by the dotted line in FIG. 2). This decreases the amount of the refrigerant jetted from the economizer port 42. Consequently, it becomes impossible to fulfill a cooling effect by the refrigerant and to make best use of the economizer effect.